Various methods are known of extracting precious minerals from mined ores. Heap leaching and vat leaching are examples of such methods.
In the heap or vat leaching processes, the mined ore, or previously comminuted tailings, containing desired metal values such as, for example gold and silver, is usually crushed to particles of small diameter; some sandy materials, however, may not require crushing. The ore, comminuted if required, is then mixed with lime and/or cement plus a solution (usually water, or barren or pregnant solution from the leaching process) to form an agglomerate. The agglomerate can also be formed by mixing the ore with a sodium hydroxide solution or with water alone. In the case of heap leaching processes, the agglomerate is sometimes placed directly on a pad and allowed to cure. Alternatively, the agglomerate may be stacked in cones and allowed to cure prior to being placed on the pad. In vat leaching, the mixture is placed into a vat and allowed to cure although it may be cured prior to being placed in the vat. Curing time is characteristic to the type of ore being processed and may range from hours to days. Research has demonstrated that heat accelerates the curing process. Following the curing phase, a leaching solution is applied. The leaching solution, for example a cyanide solution, is normally sprayed onto the agglomerate and allowed to percolate through, in the course of which the desired metal values are leached out. The leach extract is then collected and the minerals are separated. The solution is then recycled for another application to the agglomerate. Heap leach methods, as described above, are known in the art. U.S. Pat. Nos. 4,256,706 (issued Mar. 17, 1981) and 4,961,777 (issued Oct. 9, 1990) are examples of prior art pertaining to heap leaching of gold and silver containing ores.
In heap or vat leach operations it has been found that metal recoveries are considerably lower in the winter season due to lower temperatures. In the case of vat leaching during winter months, the ore entering the plant is normally cold, thus requiring longer agglomerate curing times and resulting in weak cured agglomerates. When the leaching solution is applied to such weak agglomerates, they have been found to deform, or slump resulting in "channelling" of the leach solution through the agglomerate, thereby reducing the yield since the surface area of ore contacted by the leach solution is reduced. Further, such "weak" agglomerates retard the flow of the solution, which increases the cycle time, and increase the solution turbidity, which reduces recoveries. Further, high turbidity levels result excessive clarifier maintenance and increases the probability of turbid material passing beyond the clarifier and impairing the process downstream. Another problem associated with cold agglomerates lies in the finding that the leaching solution does not progress towards the optimal leaching temperature. In the case of cyanide leaching of gold, the optimum temperature has been determined as being 85.degree. C.; however, the highest solution temperature achieved without additional heat and during the summer months is 24.degree. C. and the highest solution temperature with added heat during the colder months is in the range of 30.degree. C. The maximum operable working temperature for plant leach solution has yet to be determined; however, it is known that this value lies somewhere between 30.degree. C. and 85.degree. C. Condensation inside the building becomes a factor in making this determination.
In order to overcome the problems associated with inefficient agglomeration, attempts were made to heat the leach solution as opposed to the ore. This approach was discarded as it would not have any effect on the curing time or quality of the agglomerate since the solution is applied after the agglomeration phase. Further, engineering estimates indicated that the cost of heating the solution would be much higher than that required to heat the ore in order to achieve the same result.